That process is driven by carefully timed electrical signals that may have been disrupted by the blow to Hamlin’s chest, they said. The quickest way to get his heart back to a normal rhythm would be to shock him with an automated external defibrillator, which would also detect the abnormal rhythm, they said.

“The focus is on restoring electrical synchrony…putting it back into a normal rhythm,” said Rajesh Dash, associate professor of cardiovascular medicine at Stanford University School of Medicine.

If Hamlin was particularly unlucky, the stroke could have occurred at a particularly vulnerable moment in the heart’s electrical cycle, triggering a condition known as “commotio cordis,” according to Gregory Marcus, a cardiologist and professor of medicine at the University of California, San Francisco.

This window is only 40 milliseconds long, making the condition rare. It most commonly occurs when young baseball or hockey players receive a hit in the center of their chest from a baseball or hockey puck.

While an arrhythmia is the most likely cause of the injury that put Hamlin in critical condition, it’s not the only possibility, Dash and Marcus said. The violent contact could have resulted in, among other things, rupturing a bulge in a blood vessel called an aneurysm, or combined with a previously undetected heart defect to trigger a life-threatening event, they said.

Now the critical question is how long Hamlin’s brain went without the oxygen carried to the organs by blood flow, Marcus said. Brain tissue dies quickly when deprived of oxygen.

Medical personnel at the Bills-Cincinnati Bengals game performed cardiopulmonary resuscitation, or CPR, on Hamlin, repeatedly and rapidly pressing his chest to engage his heart and pump blood through his system.

It remains to be seen how effective these efforts have been. According to the Centers for Disease Control and Prevention website, “About 9 in 10 people who have cardiac arrest die outside of the hospital. But CPR can help improve those odds. When performed in the first few minutes after cardiac arrest, CPR can double or triple a person’s chance of survival.”

Marcus said the high death rate cited by the CDC most likely includes many older people with other serious illnesses that lead to cardiac arrest. As a young, healthy athlete, Hamlin’s chances are probably better, he said.

Hamlin, who is in critical condition at a Cincinnati hospital, is also at risk of having his heart back out of rhythm, Dash said.

“It’s a delicate time, it can change at any moment,” he said.